Optical scanning apparatus and image-forming apparatus using the same

ABSTRACT

An optical scanning apparatus includes a light source and a structure for performing light-power control for the light source, the structure including a light-power-detection optical unit that establishes an optically conjugate relationship between a deflecting surface of a deflecting unit and a light-receiving surface of a light-power detector in a main-scanning plane. Accordingly, the storage time of a light beam on the light-power-detection optical unit is increased and variation in the power of light emitted by the light source due to heat generated by the light source and environmental variation is accurately detected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical scanning apparatus suitablefor use in a laser beam printer, a digital copy machine, a multifunctionprinter, etc., that perform electrophotography processes, and animage-forming apparatus including the optical scanning apparatus.

2. Description of the Related Art

In a conventional optical scanning apparatus for a laser beam printer, acopy machine, etc., a light beam that is optically modulated inaccordance with an image signal is emitted from a light source, isperiodically deflected and scanned by a rotating polygon mirror, and isguided toward the surface of a recording medium (photosensitive drum).

The light beam deflected and scanned by a deflecting surface of thepolygon mirror is caused to form a spot on the surface of thephotosensitive recording medium (photosensitive drum) by an fθ lens.Thus, image recording is performed by optically scanning the surface ofthe recording medium.

FIG. 10 is a schematic diagram illustrating the main portion of aconventional optical scanning apparatus.

Referring to FIG. 10, a divergent light beam is emitted form a lightsource 1 and is collimated by a collimating lens 2. Then, the collimatedlight beam is incident on a cylindrical lens 4 having a predeterminedrefractive power only in a sub-scanning direction after the width of thelight beam is restricted by a diaphragm 3.

The collimated light beam incident on the cylindrical lens 4 exits thecylindrical lens 4 without a change in a main-scanning cross section(plane).

The light beam converges in a sub-scanning cross section (plane),thereby forming a line image on a deflecting surface (reflectivesurface) 5 a of the polygon mirror 5.

The light beam is deflected and scanned by the deflecting surface 5 a ofthe polygon mirror 5 and is guided toward a photosensitive drum surface8, which functions as a surface to be scanned, through an fθ lens 6. Thepolygon mirror 5 is rotated in the direction shown by the arrow A, sothat the photosensitive drum surface 8 is optically scanned in thedirection shown in the figure and image information is recorded thereon(refer to, for example, Japanese Patent Laid-Open No. 04-321370 andJapanese Patent Laid-Open No. 2002-40350).

In general, a semiconductor laser capable of direct modulation is usedas a light source for an optical scanning apparatus.

Power of light emitted from the semiconductor laser varies depending onheat emitted from the semiconductor laser itself and environmentalvariation (for example, ambient temperature variation).

Therefore, generally, the power of light emitted is constantly detectedand automatic power control is performed such that the power of thelight beam emitted from a light-emitting portion of the light source isalways maintained constant.

Various automatic power control methods for the semiconductor laser aresuggested and put into practical use.

For example, in a first method, which is most commonly used, a rearlight beam that is emitted from a semiconductor laser in a directionopposite to the direction in which an image-drawing light beam isemitted (i.e., a light beam emitted from a rear side of a semiconductorsubstrate) is detected and used for the light-power control.

The image-drawing light beam is a light beam used for forming dots in animage effective area on the photosensitive drum surface.

According to the first method, a photosensor, which functions as alight-power detector, can be installed in a package of the laser lightsource. Therefore, the overall size is relatively small and thelight-power control for the light source can be easily performed.

However, since a light beam other than the image-drawing light beam ismonitored by the photosensor and the influence of heat emitted by thelight source is significant, it is difficult to perform high-accuracylight-power control (automatic power control).

In addition, it is difficult to apply the first method to a light sourcelike a vertical cavity surface emitting laser (VCSEL) that does not emita rear light beam.

On the other hand, recently, the vertical cavity surface emitting laserhas been attracting attention as a light source for an optical scanningapparatus. Compared to a conventional edge emitting laser, the verticalcavity surface emitting laser is characterized in that the number oflight-emitting points can be considerably increased, two-dimensionalparallel integration is possible, and the layout of the light-emittingpoints is easy.

In the vertical cavity surface emitting laser, light is emitted in adirection perpendicular to the semiconductor substrate. Therefore, therear light beam is basically not emitted and it is difficult to use thelight-power control method in which the rear light beam is used.

Japanese Patent Laid-Open No. 04-321370 discusses a second method inwhich the light-power control is performed using a portion of a lightbeam emitted from a semiconductor laser that is blocked by an aperturediaphragm.

According to the method discussed in Japanese Patent Laid-Open No.04-321370, light-power control (automatic power control) can beperformed without being affected by heat emitted from the light source.

However, as the utilization ratio of the image-drawing light beam isincreased, the power of light that can be used for the automatic powercontrol is reduced in inverse proportion to the utilization ratio.

In addition, although the light used for the automatic power control isnot the rear light beam used in the first method, the blocked lightbeam, which is different from the image-drawing light beam in practice,is used and it is still difficult to perform high-accuracy light-powercontrol.

Japanese Patent Laid-Open No. 2002-40350 discusses a third method inwhich light-power control (automatic power control) is performed byseparating a portion of a light beam that travels from a light source toa deflecting unit with a half mirror and guiding the separated portiontoward a light-receiving element (photosensor).

According to the method discussed in Japanese Patent Laid-Open No.2002-40350, a portion of the actual image-drawing light beam is used.Therefore, high-accuracy automatic power control can be performed.

However, since a portion of the image-drawing light beam is separated, aloss occurs in the power of the image-drawing light beam.

In addition, an expensive optical element, such as a half mirror, isrequired for separating a portion of the image-drawing light beam.

In particular, when the above-described vertical cavity surface emittinglaser is used, high-power light emission is basically difficult comparedto the case in which the edge emitting laser is used. Therefore, theloss in the power of the image-drawing light beam caused when a portionof the image-drawing light beam is separated and detected by thelight-receiving element is a serious problem.

The output of the edge emitting laser is generally several tens ofmilliwatts, whereas the output of the vertical cavity surface emittinglaser is generally less than several milliwatts. Therefore, when thevertical cavity surface emitting laser is used, the loss in the power ofthe image-drawing light beam is a serious problem.

SUMMARY OF THE INVENTION

In light of the above-described situation, the present invention isdirected to an inexpensive, simple automatic power control method withwhich light-power control can be performed without causing loss in powerof image-drawing light beam.

According to at least one embodiment of the present invention, thelight-power control is performed using an image-drawing light beam thatis actually used, so that a vertical cavity surface emitting laser,which is a low-output light source, can be used.

In addition, according to at least one embodiment of the presentinvention, the light-power control is performed using an image-drawinglight beam that is actually used, so that high-accuracy light-powercontrol can be performed. Accordingly, high-speed, high-definitionoptical scanning apparatus and image-forming apparatus can be obtainedand the size and cost thereof can be reduced.

According to a first aspect of the present invention, there is providedan optical scanning apparatus including a light source that emits alight beam; a deflecting unit having a deflecting surface that deflectsand scans the light beam emitted from the light source; a light-powerdetector that detects the intensity of the light beam deflected andscanned by the deflecting surface of the deflecting unit; an imagingoptical unit that focuses the light beam deflected and scanned by thedeflecting surface of the deflecting unit on a surface to be scanned; alight-power-detection optical unit for guiding the light beam deflectedand scanned by the deflecting surface of the deflecting unit toward thelight-power detector; and an automatic power controller that controlsthe intensity of the light beam emitted from the light source on thebasis of a signal obtained from the light-power detector.

According to the first aspect of the present invention, thelight-power-detection optical unit establishes an optically conjugaterelationship between the deflecting surface of the deflecting unit and alight-receiving surface of the light-power detector in the main-scanningplane.

Therefore, the storage time of the light beam on thelight-power-detection optical unit is increased and variation in thepower of light emitted by the light source due to heat generated by thelight source and environmental variation is accurately detected.

As a result, the power of light emitted from the light-emitting portionsof the light source can be maintained constant.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a main-scanning sectional view of an optical scanningapparatus according to a first embodiment of the present invention.

FIG. 2 is a main-scanning sectional view of an incident optical unit anda light-power detector included in the optical scanning apparatusaccording to the first embodiment of the present invention.

FIG. 3 is a sub-scanning sectional view of the incident optical unit andthe light-power detector included in the optical scanning apparatusaccording to the first embodiment of the present invention.

FIG. 4 is a diagram illustrating the manner in which a light beam on thelight-power detector moves in response to the rotation of a deflectingunit according to the first embodiment of the present invention.

FIG. 5 is a diagram illustrating the manner in which a light beam on asynchronization detector moves in response to the rotation of adeflecting unit according to the first embodiment of the presentinvention.

FIG. 6 illustrates an image-forming apparatus according to a thirdembodiment of the present invention.

FIG. 7 is a main-scanning sectional view of an incident optical unit anda light-power detector included in an optical scanning apparatusaccording to a second embodiment of the present invention.

FIG. 8 is a sub-scanning sectional view of the incident optical unit andthe light-power detector included in the optical scanning apparatusaccording to the second embodiment of the present invention.

FIG. 9 is a diagram illustrating the manner in which a light beam on thelight-power detector moves in response to the rotation of a deflectingunit according to the second embodiment of the present invention.

FIG. 10 is a perspective view of a conventional optical scanningapparatus.

FIG. 11 is a main-scanning sectional view of the optical scanningapparatus according to the second embodiment of the present invention.

FIG. 12 is a time chart of light-power detection according to the firstembodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

First, the definition of automatic power control (APC) according toembodiments of the present invention will be described.

The automatic power control (light-power control) is performed tostabilize the output of a light beam (laser beam), which is used forraster scanning a photosensitive drum surface to form an electrostaticlatent image, with respect to temperature variation. The light beam(laser beam) is output from a light-emitting element of a light sourceand is detected (for example, once every horizontal scanning) by alight-power detector, and the thus obtained output signal is fed back toa laser driving circuit. Accordingly, the intensity (light-power) of thelight beam (laser beam) is controlled such that the output of the lightbeam (laser beam) is maintained constant at a predetermined set value(refer to, for example, Japanese Patent Laid-Open No. 05-30314).

First Embodiment

FIG. 1 is a sectional view taken along a main-scanning direction(hereafter called a main-scanning sectional view) of an optical scanningapparatus according to a first embodiment of the present invention.

FIG. 2 is a main-scanning sectional view of an incident optical systemand a light-power-detecting optical system included in the opticalscanning apparatus according to the first embodiment. FIG. 3 is asectional view taken along a sub-scanning direction (hereafter called asub-scanning sectional view) of the incident optical system and thelight-power-detecting optical system included in the optical scanningapparatus according to the first embodiment.

Here, the main-scanning direction refers to a direction perpendicular toa rotating axis of a deflecting unit (direction in which a light beam isscanned) and the sub-scanning direction refers to a direction parallelto the rotating axis of the deflecting unit (direction in which an imagecarrier is moved).

A semiconductor laser 1, which functions as a light source, is avertical cavity surface emitting laser and includes four light-emittingpoints.

As shown in FIG. 1, four light-emitting portions are arranged along aline, the line being disposed at predetermined angles relative to themain-scanning direction (Y direction) and the sub-scanning direction (Xdirection).

For simplicity, only one light beam (laser beam) is shown in FIG. 1.

Four divergent light beams emitted from the semiconductor laser 1 areconverted into four collimated light beams by a common collimating lens2 after the widths thereof in the main-scanning direction and thesub-scanning direction are restricted by a diaphragm 3.

Then, the four light beams are incident on a cylindrical lens 4 having arefractive power only in the sub-scanning direction and are focused on adeflecting surface of a rotating polygon mirror 5, which functions as adeflecting unit, in the sub-scanning direction. In the main-scanningdirection, the collimated light beams are incident on the reflectivesurface of the polygon mirror 5 without a change.

An imaging optical system 6 establishes a conjugate relationship betweenthe deflecting surface of the polygon mirror 5 and a photosensitive drumsurface in the sub-scanning cross section (plane). Accordingly, asurface-tilt correcting system is provided in the optical scanningapparatus according to the present embodiment.

The polygon mirror 5, which functions as a deflecting unit, is rotatedby a drive unit (not shown), such as a motor, at a constant speed in thedirection shown by the arrow.

The four light beams deflected and scanned by the polygon mirror 5 areincident on the imaging optical system 6 having an fθ characteristic.

In the present embodiment, the imaging optical system 6 includes twotoric lenses 61 and 62 composed of plastic.

After the fθ characteristic is provided and the field curvatures in themain-scanning direction and the sub-scanning direction are corrected,the four light beams are guided toward a photosensitive drum surface 8,which functions as a surface to be scanned.

The photosensitive drum surface 8 is optically scanned in the +Ydirection by rotating the polygon mirror 5 in the direction shown by thearrow.

Thus, scanning lines are formed on the photosensitive drum surface 8 andimage recording is performed.

In the light beams deflected by the polygon mirror 5 which functions asa deflecting unit, portions of the light beams that travel toward anarea outside an image effective area are guided toward a synchronizationdetection sensor 72 through a synchronization-detection optical element71.

The synchronization detection sensor 72 outputs a synchronizationdetection signal for determining timing for writing an image.

Similarly, in the light beams deflected by the polygon mirror 5 whichfunctions as a deflecting unit, portions of the light beams that traveltoward an area outside the image effective area are guided toward alight-power detection sensor 92, which functions as a light-powerdetector, thought a light-power detection optical element 91.

The light-power detection sensor 92 outputs a signal for controlling thepower of light emitted from the light source (automatic power control).

Thus, according to the present invention, the light beams are deflectedby the deflecting unit, and then the light-power control for the lightsource 1 is performed using the deflected light beams. Accordingly, thefollowing characteristics are provided.

(1) Since the rear light beam is not used and the powers of light beamsequivalent to the actual image-drawing light beams are detected,high-accuracy light-power control can be performed which can deal withchange in laser characteristics, including far-field pattern (FFP), dueto environmental variation (for example, ambient temperature variation).

(2) Since portions of the image-drawing light beams are not separatedusing a separating element, such as a half mirror, for light-powerdetection, loss in the light-power does not occur during the light-powerdetection.

Next, the light source according to the present embodiment will bedescribed below.

As described above, the light source according to the present embodimentis a vertical cavity surface emitting laser (VCSEL) having fourlight-emitting points that are arranged adjacent to each other.

In the vertical cavity surface emitting laser, light is emitted in adirection perpendicular to the semiconductor substrate. Therefore,compared to a conventional edge emitting laser, the number oflight-emitting points can be considerably increased, two-dimensionalparallel integration is possible, and the layout of the light-emittingpoints is easy.

However, in the surface emitting laser, the power of light emitted fromeach light-emitting point is small compared to that of the edge emittinglaser. In addition, unlike the edge emitting laser, the rear light beamis not emitted. Therefore, the light-power control is difficult.

Accordingly, when the surface emitting laser is used, the followingstructure is provided in the light-power control method (automatic powercontrol method) according to the present invention.

The role of a light-power controller 93 (automatic power controller) anda light-power detection optical system will be described in comparisonwith a synchronization-detection optical system.

To perform synchronization detection, the synchronization detectionsensor 72 can be scanned with light at a high speed and the light beamscan be focused on the synchronization detection sensor 72 or on thesynchronization detection slit 73 disposed in front of thesynchronization detection sensor 72 at least in the main-scanningdirection.

Therefore, the synchronization-detection optical element 71 focuses thecollimated light beams from the polygon mirror 5 on the synchronizationdetection sensor 72 or on the slit 73 disposed in front of thesynchronization detection sensor 72 in the main-scanning direction.

In the sub-scanning direction, focusing is not particularly importantfor the synchronization detection. Therefore, the power of thesynchronization-detection optical element 71 is set such that all of thedivergent light from the polygon mirror is incident on a light-receivingsurface of the synchronization detection sensor 72.

To perform the light-power control for the light source, a storage timeat the light-power detection sensor 92 must be a predetermined time ormore, and the light beams can be stationary on the light-power detectionsensor 92 during that time.

Therefore, in the present embodiment, the light-power detection opticalelement 91 is disposed so as to establish a conjugate relationshipbetween the deflecting surface of the polygon mirror 5 and thelight-receiving surface of the light-power detection sensor 92 in themain-scanning direction.

In FIG. 2, the solid line shows the actual light beam and the dashedlines show the conjugate (imaging) relationship.

In the main-scanning direction, the collimated light beams from thepolygon mirror 5, which functions as a deflecting unit, are once causedto converge at a position between the light-power detection opticalelement 91 and the light-power detection sensor 92 by the light-powerdetection optical element 91, and are then incident on the light-powerdetection sensor 92 in the form of divergent light.

At this time, as described above, a conjugate relationship isestablished between the deflecting surface of the deflecting unit andthe light-receiving surface in the main-scanning direction. Therefore,even when the deflecting surface of the deflecting unit rotates, thelight beams incident on the light-power detection sensor 92 arestationary unless the light beams are displaced from the light-powerdetection optical element 91.

Accordingly, in the main-scanning direction, as long as the light beamsare incident on the light-power detection optical element 91, the lightbeams are stationary on the light-receiving surface of the light-powerdetection sensor 92.

With regard to the sub-scanning direction, since the divergent lightbeams are emitted from the polygon mirror 5, a conjugate relationship isestablished between the deflecting surface and the light-receivingsurface of the light-power detection sensor 92 and the light beams arecaused to converge on the light-receiving surface, similar to themain-scanning direction.

Therefore, the light beams form line images that extend in themain-scanning direction on the light-receiving surface of thelight-power detection sensor 92, similar to the deflecting surface ofthe polygon mirror 5.

The light-power detection sensor 92 detects the intensities (powers) ofthe light beams on the light-receiving surface thereof, and outputsintensity signals to the light-power controller 93 (automatic powercontrol circuit).

Then, the light-power controller 93 (automatic power control circuit)outputs intensity correction signals to four light-emitting portions 1a, 1 b, 1 c, and 1 d in the light source 1 so that the intensities(powers) of the light beams emitted from the four light-emittingportions 1 a, 1 b, 1 c, and 1 d are maintained as a predetermined setvalue.

In the present embodiment, the cylindrical lens 4, the light-powerdetection optical element 91, and the synchronization-detection opticalelement 71 are composed of plastic and are formed integrally byinjection molding.

In addition, the semiconductor laser 1, the light-power detection sensor92, and the synchronization detection sensor 72 are arranged on the samesubstrate. Thus, the light-power control for the light source can beperformed using a small, inexpensive structure. The light-powerdetection optical element may also be formed integrally with othercomponents, such as the collimating lens, the diaphragm, etc.

Table 1 shows the optical design values of the structure along theoptical path from the light source to the light-power detector via thedeflecting unit.

TABLE 1 Optical Arrangement No. Ry Rz Asph D Glass N Light-EmittingSurface of 1 1.75 Light Source 1 Cover Glass 2 ∞ 0.25 bsl7 1.51052 3 ∞13.80 Diaphragm 3 4 2.53 Collimating Lens 2 5 ∞ 3.00 lah66 1.76167 6−15.216 21.27 Cylindrical Lens 4 7 ∞ 39.935 3.00 1.52397 8 ∞ −51.63042.80 Deflecting Surface of 9 ∞ 42.00 Deflecting Unit 5 Light-powerdetection 10  11.264 k = −0.88171 3.20 1.52397 optical element 91 11 ∞47.00 Sensor Surface of Light- 12 power detection sensor 92 Anglebetween optical axis of imaging optical system 6 and optical θ −90 deg.axis of incident optical system in main-scanning direction Angle betweenoptical axis of imaging optical system 6 and optical γ −76 deg. axis oflight-power detection optical system in main-scanning direction LightSource Wavelength 790 nm Number of Light-Emitting Portions 4 (1 row × 4lines) Intervals between Light-Emitting Portions 100 μm

According to the above-described structure, the light beams arecompletely stationary on the light-power detection sensor 92 in aparaxial area.

However, when the light-power detection optical element 91 is aspherical lens, the light beams slightly move due to the sphericalaberration of the light-power detection optical element 91 in themain-scanning cross section (plane).

In the present embodiment, to reduce the movement of the light beams,the incident surface of the light-power detection optical element 91 isformed as a rotationally symmetrical aspheric surface so that thespherical aberration can be corrected.

When the intersecting point between the incident surface of thelight-power detection optical element 91 and the optical axis is theorigin, X is the optical axis direction, and h is the radial directionperpendicular to the optical axis direction, the shape of the incidentsurface is defined as follows:

$X = \frac{\frac{h^{2}}{R}}{1 + \sqrt{1 - {\left( {1 + k} \right)\left( \frac{h}{R} \right)^{2}}}}$

To achieve stable light-power detection, the length of the light-powerdetection sensor 92 in the main-scanning direction must be larger thanthe beam diameter of the light beams on the light-power detection sensor92 in the main-scanning direction.

When the movement of the light beams in the main-scanning directionremains on the light-receiving surface of the light-power detectionsensor 92, the amount of movement must also be taken into account todetermine the length of the light-power detection sensor 92. Therefore,the correction of the spherical aberration of the light-power detectionoptical element 91 is also important from the standpoint of reduction insize of the light-power detection sensor 92.

FIG. 4 illustrates the manner in which a light beam on the light-powerdetection sensor 92 moves in response to the rotation of the polygonmirror 5 according to the present embodiment.

As a reference, FIG. 5 shows the movement of a light beam on thesynchronization detection sensor 72.

In FIGS. 4 and 5, the dot-dash line shows the principal ray of the lightbeam and the solid lines show the marginal rays of the light beam in themain-scanning direction.

In FIG. 4, the two marginal rays in the main-scanning direction are anupper ray and a lower ray.

In FIGS. 4 and 5, the horizontal axis shows the deflecting angle of thelight beam (positional reference of the light-power detection sensor 92)and the vertical axis shows the arrival position of the light beam(positional reference of the light-power detection sensor 92).

With regard to the positional reference of the light-power detectionsensor 92, the optical axis of the light-power detection optical system(the optical axis of the light-power detection optical element 91) isset as a reference (zero) of the light-beam arrival position in themain-scanning cross section (plane).

In FIG. 1, with the optical axis of the light-power detection opticalsystem (the optical axis of the light-power detection optical element91) being set as a reference (zero) of the light-beam arrival position,a clockwise movement of the light beam (direction in which thelight-receiving surface is scanned by the light beam) is defined asnegative and a counterclockwise movement of the light beam (direction inwhich the light beam approaches the imaging optical system 6 in themain-scanning direction) is defined as positive.

In the main-scanning cross section (plane), an optically conjugaterelationship is established between the deflecting surface and thelight-receiving surface. Therefore, although the light beam has acertain width on the light-receiving surface, the arrival position ofthe light beam on the light-receiving surface in the main-scanningdirection barely changes even when the deflecting angle of the polygonmirror changes.

In the synchronization-detection optical system, the principal ray andthe outermost rays of the light beam coincide with one another on thelight-receiving surface of the synchronization detection sensor 72 inthe main-scanning cross section (plane). More specifically, in themain-scanning cross section (plane), the light beam is focused on thelight-receiving surface of the synchronization detection sensor 72.

However, in the main-scanning cross section (plane), the deflectingsurface of the polygon mirror 5 and the light-receiving surface of thesynchronization detection sensor 72 are not in an optically conjugaterelationship.

Therefore, as the deflecting angle of the polygon mirror 5 changes, thelight beam arrival position on the light-receiving surface of thesynchronization detection sensor 72 in the main-scanning directionlargely changes while the light beam is being incident on thesynchronization-detection optical element 71.

The scanning angular velocity Vapc on the light-power detection sensor92 in the light-power-detection optical system can satisfy the followingexpression:Vapc<f/10where f is the fθ coefficient (mm/rad) of the optical scanningapparatus.

When this parameter exceeds the upper limit, the length of thelight-receiving surface of the light-power detection sensor 92 in themain-scanning direction is increased and it becomes difficult to providea small, inexpensive light-power detection optical system.

In the present embodiment, Vapc is 3.2 (mm/rad) and f is 150 (mm/rad).Thus, the above expression is satisfied.

In the present embodiment, the light beams incident on the imagingoptical system 6 are collimated. Therefore, the fθ coefficient f of theoptical scanning apparatus is equal to the focal length of the imagingoptical system 6.

The fθ coefficient is the moving distance of the light beams on thescanned surface per unit deflecting angle of the polygon mirror 5, andindicates the scanning angular velocity of the optical scanningapparatus.

An imaging magnification βam between the deflecting surface of thedeflecting unit and the light-receiving surface of the light-powerdetection sensor 92 provided by the light-power detection opticalelement 91 in the main-scanning direction can satisfy the followingexpression:0.05<|βam|<1.5

When this parameter exceeds the upper limit, the size of thelight-receiving surface of the light-power detection sensor 92 isincreased and it becomes difficult to provide a small, inexpensivelight-power detection optical system.

When the parameter is below the lower limit, Fno of the light-powerdetection optical system is reduced and it becomes difficult to provideaberration correction of the light-power detection optical system.Therefore, it becomes difficult to keep the light beams stationary onthe light-power detection sensor 92.

In the present embodiment, |βam| is 1.27. Thus, the above expression issatisfied.

According to the present embodiment, in the structure for thelight-power control (light-beam-intensity adjustment) for the lightsource in the optical scanning apparatus, the light-power-detectionoptical unit establishes an optically conjugate relationship between thedeflecting surface of the deflecting unit and the light-receivingsurface of the light-power detection sensor 92.

Therefore, while the light beams deflected and scanned by the deflectingsurface are incident on the light-power detection optical element 91,the light beams incident on the light-receiving surface of thelight-power detection sensor 92 are optically stationary on thelight-receiving surface.

Accordingly, the storage time of the light beams on the light-powerdetector can be increased and variation in the power of light emitted bythe light source (variations in the intensities of the light beams) dueto heat generated by the light source itself and environmental variation(for example, ambient temperature variation) can be accurately detected.

Therefore, the powers of the light beams emitted from the light-emittingportions of the light source can be maintained constant.

Next, a light-power control method for controlling the powers of aplurality of light beams will be described.

In the above-described structure, the time in which the light beams canbe kept stationary on the light-receiving surface of the light-powerdetection sensor 92 is limited to the time in which the light beamsdeflected by the deflecting unit are incident on the light-powerdetection optical element 91.

Therefore, when the number of light-emitting portions in the lightsource is increased, it becomes difficult to perform the light-powercontrol by turning on all of the light-emitting portions in time-sharingin a single scanning cycle.

Therefore, in the present embodiment, the light-power control(light-beam-intensity adjustment) is performed by successively turningon the four light-emitting portions one at a time in each scanningcycle. Accordingly, the light-power control for all of thelight-emitting portions is completed after four scanning cycles.

More specifically, in the present embodiment, the light-power control(light-beam-intensity adjustment) is performed by successively turningon the four light-emitting portions one at a time in each scanning cycleusing a single deflecting surface of the polygon mirror. The light-powercontrol for all of the light-emitting portions is completed after thepolygon mirror is rotated by one turn.

In other words, the light-power adjustment is successively performed forthe light-emitting portions one at a time in each scanning cycle, sothat the light-power adjustment for all of the light-emitting portionsin the light source is completed after a plurality of scanning cycles.

A single scanning cycle corresponds to a scanning process performed by asingle surface of the polygon mirror.

This will be described in more detail below with reference to FIG. 12.

FIG. 12 is a timing chart of scanning lines drawn for the automaticpower control (APC) and synchronization detection and those drawn in theimage effective area on the photosensitive drum surface 8.

As shown in FIG. 12, the automatic power control (APC) and thesynchronization detection for controlling the light-emitting portions ofthe light source 1 are performed in that order before the four scanninglines 1 a, 1 b, 1 c, and 1 d are drawn in the image effective area usinga first surface of the deflecting unit 5 (four-surface polygon mirror).

When the first surface of the deflecting unit 5 (four-surface polygonmirror) is used, only the light-emitting portion that emits the lightbeam corresponding to the scanning line 1 a is subjected to theautomatic power control (light-power adjustment).

In the synchronization detection, only the light beam corresponding tothe scanning line 1 a is detected by the synchronization detectionsensor 72. With regard to the other three light beams corresponding tothe scanning lines 1 b, 1 c, and 1 d, write start positions on thephotosensitive drum surface 8 in the main-scanning direction (writestart times) are determined on the basis of the synchronizationdetection signal obtained by detecting the light beam corresponding tothe scanning line 1 a. The write start position on the photosensitivedrum surface 8 in the main-scanning direction (write start time) for thelight beam corresponding to the scanning line 1 a is, of course, alsodetermined by the synchronization detection signal obtained by detectingthe light beam corresponding to the scanning line 1 a.

Next, when a second surface of the deflecting unit 5 (four-surfacepolygon mirror) is used, only the light-emitting portion that emits thelight beam corresponding to the scanning line 1 b is subjected to theautomatic power control (light-power adjustment).

Then, when a third surface of the deflecting unit 5 (four-surfacepolygon mirror) is used, only the light-emitting portion that emits thelight beam corresponding to the scanning line 1 c is subjected to theautomatic power control (light-power adjustment).

Then, when a fourth surface of the deflecting unit 5 (four-surfacepolygon mirror) is used, only the light-emitting portion that emits thelight beam corresponding to the scanning line 1 d is subjected to theautomatic power control (light-power adjustment).

In the synchronization detection, only the light beam corresponding tothe scanning line 1 a is detected by the synchronization detectionsensor 72 for all of the first to fourth surfaces. With regard to theother three light beams corresponding to the scanning lines 1 b, 1 c,and 1 d, write start positions on the photosensitive drum surface 8 inthe main-scanning direction (write start times) are determined on thebasis of the synchronization detection signal obtained by detecting thelight beam corresponding to the scanning line 1 a.

The write start position on the photosensitive drum surface 8 in themain-scanning direction (write start time) for the light beamcorresponding to the scanning line 1 a is, of course, also determined bythe synchronization detection signal obtained by detecting the lightbeam corresponding to the scanning line 1 a.

According to the present invention, the light-power control can beperformed by the above-described sequence for the case in which thenumber of light-emitting portions is increased.

It is clear that the effects of the present invention can also beobtained when the four light-emitting portions are divided into twopairs and the light-power control is successively performed one pair ata time. Thus, the present invention is not limited to theabove-described sequence.

Second Embodiment

FIG. 11 is a main-scanning sectional view of an optical scanningapparatus according to a second embodiment of the present invention.

FIG. 7 is a main-scanning sectional view of an incident optical systemincluding components 2, 3, and 4 and a light-power detection opticalsystem included in the optical scanning apparatus according to thesecond embodiment of the present invention.

FIG. 8 is a sub-scanning sectional view of the incident optical systemincluding components 2, 3, and 4 and the light-power detection opticalsystem included in the optical scanning apparatus.

The present embodiment differs from the first embodiment in that an edgeemitting monolithic multilaser is used as a light source 1 and theimaging magnification of the light-power detection optical system isreduced. Other structures of the present embodiment are similar to thoseof the first embodiment.

The semiconductor laser 1, which functions as a light source, is an edgeemitting monolithic multilaser including two light-emitting portions.

These two light-emitting points are arranged along respective lines thatare disposed at predetermined angles relative to the main-scanningdirection and the sub-scanning direction (see FIG. 11).

In the present embodiment, only one light beam is shown in FIGS. 11, 7,and 8 for simplicity.

Two divergent light beams output from two light-emitting portions of thesemiconductor laser 1 are converted into two collimated light beams by acommon collimating lens 2 after the widths thereof in the main-scanningdirection and the sub-scanning direction are restricted by a diaphragm3.

Then, the two light beams are incident on a cylindrical lens 4 having arefractive power only in the sub-scanning direction, and are focused ona reflective surface of a rotating polygon mirror 5 in the sub-scanningcross section.

In the main-scanning direction, the two collimated light beams areincident on the reflective surface of the polygon mirror 5 (four-surfacepolygon mirror) without a change.

The polygon mirror 5, which functions as a deflecting unit, is rotatedby a drive unit (not shown), such as a motor, at a constant speed in thedirection shown by the arrow.

Similar to the first embodiment, the two light beams deflected andscanned by the polygon mirror 5 are incident on an imaging opticalsystem 6 having an fθ characteristic, and are then guided toward aphotosensitive drum surface 8, which is a surface to be scanned. Thus,image recording is performed.

The imaging optical system 6 establishes an optically conjugaterelationship between the deflecting surface of the polygon mirror 5 anda photosensitive drum surface in the sub-scanning cross section.Accordingly, a surface-tilt correcting system is provided in the opticalscanning apparatus according to the present embodiment.

In the present embodiment, the imaging optical system 6 includes twotoric lens 61 and 62 composed of plastic.

After the fθ characteristic is provided and the field curvatures in themain-scanning direction and the sub-scanning direction are corrected,the two light beams are guided toward the photosensitive drum surface 8.

The photosensitive drum surface 8 is optically scanned in +Y directionby rotating the polygon mirror 5 in the direction shown by the arrow.

In the light beams deflected by the polygon mirror 5 which functions asa deflecting unit, portions of the light beams that travel toward anarea outside an image effective area are guided toward a synchronizationdetection sensor 72 through a synchronization-detection optical element71.

The synchronization detection sensor 72 outputs a synchronizationdetection signal for determining timing for writing an image (writestart position in the main-scanning direction).

Similarly, in the light beams deflected by the polygon mirror 5 whichfunctions as a deflecting unit, portions of the light beams that traveltoward an area outside an image effective area are guided toward alight-power detection sensor 92, which functions as a light-powerdetector, through a light-power detection optical element 94.

The light-power detection sensor 92 outputs a signal for controlling thepower (intensity) of light emitted from the light source.

Thus, also in the present embodiment, light beams are deflected by thedeflecting unit 5, and then the light-power control for the light sourceis performed using the deflected light beams.

The light-power detection sensor 92 detects the intensities (powers) ofthe light beams on the light-receiving surface thereof, and outputsintensity signals to a light-power controller 93 (automatic powercontrol circuit).

Then, the light-power controller 93 (automatic power control circuit)outputs intensity correction signals for the two light-emitting portions1 a and 1 b in the light source 1, which is an edge emitting laser, sothat the intensities (powers) of the light beams emitted from the twolight-emitting portions 1 a and 1 b are maintained as a predeterminedset value.

Next, the light source used in the present embodiment will be describedbelow.

As described above, the light source 1 according to the presentembodiment is an edge emitting laser having two light-emitting pointsthat are arranged adjacent to each other.

In the edge emitting laser, light is emitted in a directionperpendicular to an end face of the substrate and the output power canbe easily increased compared to a surface emitting laser.

A rear light beam is emitted from the other end face of the substrate,and the light-power control can also be performed by directly monitoringthe rear light beam.

However, in this case, the light-power control (automatic power control)is performed using a light beam that is not actually used for drawing animage. In addition, the light-receiving surface is easily affected byheat generated by the edge emitting laser itself. Therefore, it isdifficult to perform high-accuracy light-power control.

Next, the light-power detection optical system will be described indetail below with reference to FIG. 7.

In order to keep the light beams stationary on the light-power detectionsensor 92 for a predetermined time, the light-power detection opticalelement 94 is disposed so as to establish an optically conjugaterelationship between the deflecting surface of the deflecting unit 5 andthe light-receiving surface of the light-power detection sensor 92 inthe main-scanning direction.

In other words, while the light beams deflected by the deflectingsurface are incident on the light-power detection optical element 94,the light beams incident on the light-receiving surface of thelight-power detection sensor 92 are optically stationary on thelight-receiving surface.

Accordingly, the storage time of the light beams on the light-powerdetector 92 can be increased and variation in the power of light emittedby the light source 1 due to heat generated by the light source 1 itselfand environmental variation (for example, ambient temperature variation)can be accurately detected.

Therefore, the powers of the light beams emitted from the light-emittingportions of the light source 1 can be maintained constant.

In FIG. 7, the solid lines show the actual light beams.

Referring to FIG. 7, in the main-scanning cross section, the collimatedlight beams deflected and scanned by the deflecting surface are oncefocused at a position between the light-power detection optical element94 and the light-power detection sensor 92, and are then incident on thelight-receiving surface of the light-power detection sensor 92.

The dashed lines show that the deflecting surface of the polygon mirror5 and the light-receiving surface of the light-power detection sensor 92are in the optically conjugate relationship in the main-scanning crosssection.

In the main-scanning direction, the collimated light beams from thepolygon mirror 5, which functions as a deflecting unit, are caused toconverge at a position between the light-power detection optical element94 and the light-power detection sensor 92 by the light-power detectionoptical element 94, and are then incident on the light-receiving surfaceof the light-power detection sensor 92 in the form of the divergentlight.

Since an optically conjugate relationship is established between thedeflecting surface of the deflecting unit 5 and the light-receivingsurface of the light-power detection sensor 92, even when the deflectingsurface of the deflecting unit rotates, the light beams incident on thelight-power detection sensor 92 are stationary unless the light beamsare displaced from the light-power detection optical element 94.

Since the divergent light beams are emitted from the polygon mirror 5also in the sub-scanning direction, an optically conjugate relationshipis also established between the deflecting surface of the polygon mirror5 and the light-receiving surface of the light-power detection sensor92, so that the light beams converge on the light-receiving surface.

Therefore, the light beams form line images that extend in themain-scanning direction on the light-receiving surface, similar to thedeflecting surface.

Different from the first embodiment, in the present embodiment, thecylindrical lens 4 and the light-power detection optical element 94 areformed separately from each other. In addition, the semiconductor laser1 and the light-power detection sensor 92 are formed on differentsubstrates.

Since there is no layout restrictions, design freedom of the light-powerdetection optical system is increased. Accordingly, the sensor size canbe further reduced by reducing the moving velocity of the light beamsand the diameters of the light beams on the sensor.

In the present embodiment, the light-power control (light-beam intensityadjustment) is performed by successively turning on the twolight-emitting portions one at a time in each scanning cycle using asingle deflecting surface of the polygon mirror. Accordingly, thelight-power control for the two light-emitting portions is completedafter the polygon mirror is rotated by a half turn.

Table 2 shows the optical design values of the structure along theoptical path from the light source 1 to the light-power detection sensor92 via the deflecting unit

TABLE 2 Optical Arrangement No. Ry Rz Asph D Glass N Light-EmittingSurface of 1 1.75 Light Source 1 Cover Glass 2 ∞ 0.25 bsl7 1.51052 3 ∞13.80 Diaphragm 3 4 2.53 Collimating Lens 2 5 ∞ 3.00 lah66 1.76167 6−15.216 21.27 Cylindrical Lens 4 7 ∞ 39.935 3.00 1.52397 DeflectingSurface of 8 ∞ −51.630 42.80 Deflecting Unit 5 9 ∞ 20.00 Light-powerdetection optical 10  3.747 k = −0.55699 3.00 1.52397 element 91 11 ∞10.00 Sensor Surface of Light- 12 power detection sensor 92 Anglebetween optical axis of imaging optical system 6 and optical θ −90 deg.axis of incident optical system in main-scanning direction Angle betweenoptical axis of imaging optical system 6 and optical γ −70 deg. axis oflight-power detection optical system in main-scanning direction LightSource Wavelength 790 nm Number of Light-Emitting Portions 2 (1 row × 2lines) Intervals between Light-Emitting Portions 100 μm

According to the above-described structure, the light beams arecompletely stationary in the main-scanning direction on thelight-receiving surface of the light-power detection sensor 92 in aparaxial area.

However, the light beams slightly move on the light-receiving surface inpractice due to the spherical aberration of the light-power detectionoptical element 94 in the main-scanning cross section.

Also in the present embodiment, to reduce the movement of the lightbeams on the light-receiving surface in the main-scanning direction, theincident surface of the light-power detection optical element 94 isformed as a rotationally symmetrical aspheric surface so that thespherical aberration can be corrected.

In other words, the incident surface of the light-power detectionoptical element 94 has a non-arc shape in the main-scanning crosssection.

To achieve stable light-power detection, the length of thelight-receiving surface of the light-power detection sensor 92 in themain-scanning direction must be larger than the beam diameter of thelight beams on the light-power detection sensor 92 in the main-scanningdirection.

FIG. 9 illustrates the manner in which a light beam on the light-powerdetection sensor 92 moves in response to the rotation of the deflectingunit 5.

In FIG. 9, the dot-dash line shows the principal ray of the light beamand the solid lines show the marginal rays of the light beam in themain-scanning direction.

In FIG. 9, the two marginal rays in the main-scanning direction are anupper ray and a lower ray.

In FIG. 9, the horizontal axis shows the deflecting angle of the lightbeam (positional reference of the light-power detection sensor 92) andthe vertical axis shows the arrival position of the light beam(positional reference of the light-power detection sensor 92).

With regard to the positional reference of the light-power detectionsensor 92, the optical axis of the light-power detection optical system(the optical axis of the light-power detection optical element 94) isset as a reference (zero) of the light-bam arrival position in themain-scanning cross section.

In FIG. 1, with the optical axis of the light-power detection opticalsystem (the optical axis of the light-power detection optical element94) being set as a reference (zero) of the light-beam arrival position,a clockwise movement of the light beam (direction in which thelight-receiving surface is scanned by the light beam) is defined asnegative and a counterclockwise movement of the light beam (direction inwhich the light beam approaches the imaging optical system 6 in themain-scanning direction) is defined as positive.

It is clear from FIG. 9 that, similar to the first embodiment, thearrival position of the light beam barely changes even when thedeflecting angle of the polygon mirror 5 changes.

The scanning angular velocity Vapc on the light-power detection sensorin the light-power detection optical system can satisfy the followingexpression:Vapc<f/10where f is the fθ coefficient (mm/rad) of the optical scanningapparatus. When this parameter exceeds the upper limit, the size of thelight-receiving surface of the light-power detection sensor 92 isincreased and it becomes difficult to provide a small, inexpensivelight-power detection optical system.

In the present embodiment, Vapc is 1.3 (mm/rad) and f is 150 (mm/rad).Thus, the above expression is satisfied.

An imaging magnification βam between the deflecting surface of thedeflecting unit 5 and the light-receiving surface of the light-powerdetection sensor 92 provided by the light-power detection opticalelement 91 in the main-scanning direction can satisfy the followingexpression:0.05<|βam|<1.5

When this parameter exceeds the upper limit, the length of thelight-receiving surface of the light-power detection sensor 92 in themain-scanning direction is increased and it becomes difficult to providea small, inexpensive light-power detection optical system.

When the parameter is below the lower limit, Fno of the light-powerdetection optical system is reduced and it becomes difficult to provideaberration correction of the light-power detection optical system.Therefore, it becomes difficult to keep the light beams stationary onthe light-power detection sensor.

In the present embodiment, |βam|is 0.73. Thus, the above expression issatisfied.

In the above-described first and second embodiments, in the structurefor performing the automatic power control of the light source 1 in theoptical scanning apparatus, the light-power detection optical elementestablishes an optically conjugate relationship between the deflectingsurface of the deflecting unit and the light-receiving surface of thelight-power detection sensor 92 in the main scanning direction.

Accordingly, in the first and second embodiments, while the light beamsare incident on the light-power detection optical element 94, the lightbeams on the light-receiving surface of the light-power detection sensor92 are optically stationary in the main-scanning direction.

As a result, according to the first and second embodiments, aninexpensive, simple structure for performing the light-power controlwithout loss in the light-power can be provided.

In the first embodiment, a low-output light source, such as a verticalcavity surface emitting laser, can be used.

In the first end second embodiments, since the light-power control isperformed using the actual image-drawing light beam, high-accuracylight-power control can be performed which can deal with change in lasercharacteristics, including far-field pattern (FFP), due to environmentalvariation (for example, ambient temperature variation).

In the first and second embodiments, a vertical cavity surface emittinglaser including a plurality of light-emitting portions (4 beam) and anedge emitting monolithic multilaser including a plurality oflight-emitting portions (2 beam) are used. However, the presentinvention is not limited to multibeam lasers.

Similar to the case in which multibeam lasers are used, the effects ofthe present invention can also be obtained when a single-beam laserhaving a single light-emitting portion is used as the light source.

According to the present invention, the number of light-emittingportions of the multibeam laser is not limited as long as two or morelight-emitting portions are provided. Since there is a growing demandfor high-speed processes, the structure of the present invention can beeffectively used in multibeam lasers that emit four or more light beams.

The reason for this is because in either of the vertical cavity surfaceemitting laser and the edge emitting monolithic multilaser, the outputfrom each light-emitting portion is reduced as the number oflight-emitting portions is increased.

In addition, in the first and second embodiments, the imaging opticalsystem 6 includes two toric lenses 61 and 62. However, the presentinvention is not limited to this. According to the present invention,the imaging optical system 6 may also be composed of a single toriclens. In addition, the imaging optical system 6 may also be composed ofthree or more lenses. In addition, according to the present invention,the imaging optical system 6 may include a curved mirror or adiffractive optical element.

Third Embodiment

FIG. 6 is a schematic diagram illustrating the main portion of a colorimage-forming apparatus according to a third embodiment of the presentinvention.

Referring to FIG. 6, a color image-forming apparatus 160 includesoptical scanning apparatus 110, which each have the structure accordingto the above-described first embodiment, photosensitive drums 121, 122,123 and 124 which each can function as an image carrier, developingdevices 131, 132, 133 and 134, and a conveying belt 151.

Referring to FIG. 6, the color image-forming apparatus 160 receives red(R), green (G), and blue (B) signals from an external device 152, suchas a personal computer.

These signals are respectively converted into image data elements (dotdata elements) 141, 142, 143, and 144 for cyan (C), magenta (M), yellow(Y), and black (K), respectively, by a printer controller 153 includedin the color image-forming apparatus 160.

The image data elements are input to the corresponding optical scanningapparatuses 110.

Each of the four optical scanning apparatuses 110 emits four light beamsthat are modulated in accordance with the corresponding image dataelement, and photosensitive surfaces of the photosensitive drums 121,122, 123 and 124 are scanned in the main scanning direction by the 4×4light beams.

In the color image-forming apparatus 160 according to the presentembodiment, the 4×4 light beams emitted from the optical scanningapparatuses 110 in accordance with the respective image data elementsare used to form latent images of the respective colors on thecorresponding photosensitive drums 121, 122, 123, and 124. Then, theimages are superimposed on a recording medium to obtain a single, fullcolor image.

Reference numerals 131, 132, 133, and 134 denote developing devices. Asdescribed above, the light beams are modulated on the basis of the imagedata elements, and the surfaces of the photosensitive drums 121, 122,123 and 124 are irradiated with the light beams so that electrostaticlatent images are formed on the surfaces thereof. The electrostaticlatent images are developed as toner images by the developing devices131, 132, 133, and 134 disposed such that the developing devices 131,132, 133, and 134 are in contact with the photosensitive drums 121, 122,123 and 124, respectively, at positions on the downstream of thepositions at which the photosensitive drums 131, 132, 133, and 134 areirradiated with the light beams in the rotating direction of thephotosensitive drums 131, 132, 133, and 134.

The toner images developed by the developing devices 131, 132, 133, and134 are transferred onto a paper sheet that can function as atransferring material by transferring rollers (not shown) disposed belowthe photosensitive drums 121, 122, 123 and 124 so as to face thephotosensitive drums 121, 122, 123 and 124. Although the paper sheet isfed from a paper cassette disposed in front of the photosensitive drums121, 122, 123 and 124 in this example, it can also be fed manually. Apaper feed roller that is disposed at an end of the paper cassetteconveys the paper sheet contained in the paper cassette to atransporting path.

The paper sheet on which the unfixed toner image is transferred asdescribed above is further transported to a fixing device (not shown)disposed behind the photosensitive drums 121, 122, 123 and 124. Thefixing device includes a fixing roller (not shown), which can have afixing heater (not shown) therein, and a pressure roller (not shown)disposed so as to be in pressure contact with the fixing roller. Thepaper sheet conveyed from the transferring section is pressed and heatedin a nip portion between the fixing roller and the pressure roller sothat the unfixed toner image on the paper is fixed. Paper output rollers(not shown) are disposed behind the fixing roller and the paper sheet onwhich the image is fixed is output from the image-forming apparatus.

The external device 152 can include, for example, a color image readingapparatus, which can have a CCD sensor. In this case, a system includingthe color image reading apparatus and the color image-forming apparatus160 can function as a color digital copying machine.

The optical scanning apparatus according to the first embodiment is notlimited to color digital copy machines, and may also be applied to colorlaser beam printers, monochrome digital copy machines, monochrome laserbeam printers, etc.

The optical scanning apparatus according to the second embodiment canalso be applied to image-forming apparatus, such as laser beam printersand digital copy machines, that perform electrophotography processes.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications, equivalent structures and functions.

This application claims priority from Japanese Application No.2005-297617 filed Oct. 12, 2005, which is hereby incorporated byreference herein in its entirety.

1. An optical scanning apparatus comprising: a light source that emits alight beam; a deflecting unit having a deflecting surface that deflectsand scans the light beam emitted from the light source; a light-powerdetector that detects the intensity of the light beam deflected andscanned by the deflecting surface of the deflecting unit; an imagingoptical unit that focuses the light beam deflected and scanned by thedeflecting surface of the deflecting unit on a surface to be scanned; alight-power-detection optical unit for guiding the light beam deflectedand scanned by the deflecting surface of the deflecting unit toward thelight-power detector; and an automatic power controller that controlsthe intensity of the light beam emitted from the light source on thebasis of a signal obtained from the light-power detector, wherein thelight-power-detection optical unit establishes an optically conjugaterelationship between the deflecting surface of the deflecting unit and alight-receiving surface of the light-power detector in the main-scanningplane, and a scanning angular velocity Vapc on the light-receivingsurface of the light-power detector satisfies the following expression:Vapc<f/10 where f is fθ coefficient of the optical scanning apparatus inthe unit of mm/rad.
 2. The optical scanning apparatus according to claim1, wherein an imaging magnification βam between the deflecting surfaceof the deflecting unit and the light-receiving surface of thelight-power detector in the main-scanning direction satisfies thefollowing expression:0.05<|βam|<1.5.
 3. The optical scanning apparatus according to claim 1,wherein the size of the light-receiving surface of the light-powerdetector in the main-scanning direction is larger than a diameter of thelight beam guided to the light-receiving surface in the main-scanningdirection.
 4. The optical scanning apparatus according to claim 1,wherein the light source includes a surface emitting laser having aplurality of light-emitting portions.
 5. The optical scanning apparatusaccording to claim 1, wherein the light source includes an edge emittinglaser having a plurality of light-emitting portions.
 6. The opticalscanning apparatus according to claim 1, wherein the light sourceincludes a single laser beam.
 7. The optical scanning apparatusaccording to claim 1, wherein the light-power-detection optical unitestablishes an optically conjugate relationship between the deflectingsurface of the deflecting unit and the light-receiving surface of thelight-power detector in the sub-scanning plane.
 8. The optical scanningapparatus according to claim 1, wherein the light source includes amulti-beam laser having a plurality of light-emitting portions that emita plurality of light beams, the light beams being deflected and scannedby the same deflecting surface on the deflecting unit and being guidedto the light-power detector by the light-power-detection optical unit,and wherein the light beams deflected and scanned by the same deflectingsurface are guided to the light-power detector in predetermined timeintervals.
 9. The optical scanning apparatus according to claim 1,wherein at least one surface of the light-power-detection optical unithas a non-arc shape in the main-scanning cross section.
 10. Animage-forming apparatus comprising: an optical scanning apparatusaccording to claim 1; a photosensitive body having a surface to bescanned; a developing device that forms a toner image by developing anelectrostatic latent image formed on the surface of the photosensitivebody by the light beam scanned by the optical scanning apparatus; atransferring device that transfers the toner image onto a transferringmaterial; and a fixing device that fixes the toner image transferredonto the transferring material.
 11. An image-forming apparatuscomprising: an optical scanning apparatus according to claim 1; and aprinter controller that converts code data received from an externaldevice into an image signal and inputs the image signal to the opticalscanning apparatus.
 12. An image-forming apparatus comprising: anoptical scanning apparatus according to claim 1; a photosensitive bodyhaving a surface to be scanned; a developing device that forms a tonerimage by developing an electrostatic latent image formed on the surfaceof the photosensitive body by the light beam scanned by the opticalscanning apparatus; a transferring device that transfers the toner imageonto a transferring material; and a fixing device that fixes the tonerimage transferred onto the transferring material.
 13. An image-formingapparatus comprising: an optical scanning apparatus according to claim1; and a printer controller that converts code data received from anexternal device into an image signal and inputs the image signal to theoptical scanning apparatus.
 14. The optical scanning apparatuscomprising: a light source that emits a light beam; a deflecting unithaving a deflecting surface that deflects and scans the light beamemitted from the light source; a light-power detector that detects theintensity of the light beam deflected and scanned by the deflectingsurface of the deflecting unit; an imaging optical unit that focuses thelight beam deflected and scanned by the deflecting surface of thedeflecting unit on a surf ace to be scanned; a light-power-detectionoptical unit for guiding the light beam deflected and scanned by thedeflecting surface of the deflecting unit toward the light-powerdetector; and an automatic power controller that controls the intensityof the light beam emitted from the light source on the basis of a signalobtained from the light-power detector, wherein thelight-power-detection optical unit establishes an optically conjugaterelationship between the deflecting surface of the deflecting unit and alight-receiving surface of the light-power detector in the main-scanningplane, wherein the light source has a plurality of light-emittingportions, and wherein the intensity control for the light source issuccessively performed for the light-emitting portions one at a time ineach scanning cycle, so that the intensity control for all of thelight-emitting portions is completed after a plurality of scanningcycles.